Conventionally, in an engine for an automobile or the like, development of an exhaust device for the purpose of enhancing engine output is carried out. Patent Literature 1 discloses an example of the exhaust device. An exhaust device described in Patent Literature 1 includes a plurality of independent exhaust pipes respectively connected to exhaust ports of a plurality of cylinders, in which exhaust operations are not performed consecutively; and a mixing pipe having a circular cross section, connected to downstream ends of the independent exhaust pipes, and through which exhaust gas that has passed through the independent exhaust pipes flows in, wherein cross-sectional shapes of the downstream ends of the independent exhaust pipes are fan shapes identical to each other, and the downstream ends of the independent exhaust pipes are connected to an upstream end of the mixing pipe in a state that the independent exhaust pipes are gathered in such a manner that the fan shapes are formed into a circular shape.
According to the exhaust device, a negative pressure is generated within the mixing pipe when exhaust gas that has passed through the independent exhaust pipes flows into the mixing pipe. An ejector effect such that exhaust gas within other one of the independent exhaust pipes and within an exhaust port of a cylinder communicating with the other one of the independent exhaust pipes is sucked downstream by the negative pressure. Further, exhaust gas from the cylinder is promoted by the ejector effect, and engine output is enhanced.
However, in the exhaust device described in Patent Literature 1, exhaust gas flows into the mixing pipe having a circular cross section from the cross-sectional fan-shaped downstream ends of the independent exhaust pipes. Therefore, it is difficult to uniformly distribute exhaust gas flowing in through the independent exhaust pipes within the mixing pipe. Consequently, backflow of exhaust gas is likely to occur within the mixing pipe, and a suction amount of exhaust gas from the independent exhaust pipes may not be sufficiently secured.